Polyamidoamines (PAAs) are synthetic polymers obtained by Michael polyaddition, with a step mechanism, of primary or secondary amines and bisacrylamides, according to Scheme 1, which relates generically to the synthesis of PAAs from primary amines

wherein R1 is hydrogen or an optionally substituted alkyl, R2 is an alkylene, R3 is an optionally substituted alkyl, and n is an integer that expresses the number of repetitive units required to give the polymer the desired molecular weight.
PAAs and methods for their preparation are described, for example, in WO2010/099962, WO2011/145056, P. Ferruti et al., Biomacromolecules, 2007, 8, 1498-1504, and P. Ferruti, J. Polym. Sci. A1 51, 2319-2353, (2013).
The polymerisation is usually conducted in water, in concentrated solution (≥25%), at room temperature, and in times ranging from 2 to 8 days. In special cases, other solvents such as alcohols, glycerin or mixtures of dimethylsulphoxide and water can be used. However, polar monomers such as amino acids, and the PAAs deriving from them, are generally poorly soluble in said solvents. Moreover, the polymerisation rates are often lower, and in most cases water is more convenient, if it can be used. In solvents without mobile hydrogen atoms, the polymerisation rate is modest, and the molecular weights of the products obtained are low.
Agmatine or 4-aminobutylguanidine (hereinafter called “G”) is a compound deriving from decarboxylation of arginine, a natural amino acid containing a guanidine group, a very strong base, and a primary amino group of moderate basicity.
Agmatine reacts in aqueous solution with bisacrylamides, giving rise to a step polyaddition which, if the pH is maintained under 10, only involves the amino group, because under those conditions the guanidine group is protonated and does not react.
WO2011/145056 describes the copolymerisation of agmatine with 2,2-bis(acrylamido)acetic acid (Scheme 2). The resulting product, called AGMA-1, is a PAA whose side chains contain an amidino group, and which has antiviral activity.
Copolymerisation of agmatine with N,N-methylenebisacrylamide (hereinafter called “MBA”) provides PAA containing guanidine groups called MBA-G) (Scheme 2):

MBA-G is soluble in water and, like agmatine, is very basic. The inventors of the present invention have found that MBA-G possesses strong antimicrobial properties against many pathogenic micro-organisms.
A common characteristic of PAAs is that they form copolymers by copolyaddition of mixtures of different amines with the same bisacrylamide, or of the same amine with mixtures of different bisacrylamides. For example, the product obtained by copolymerising agmatine with mixtures of 2,2-bis(acrylamido)acetic acid and N,N-methylenebisacrylamide (MBA) contains both units in the same proportion as the two starting bisacrylamides, arranged randomly along the polymer chain, but both of them preceded and followed by a guanidine unit. All these copolymers are basic, but less than MBA-G, because they are internally buffered by the presence of carboxyl groups. The copolymers according to the invention (see below) constitute another example.
Among the amines usable as monomers in the synthesis of PAA, natural amino acids occupy a particular place. In their natural state they do not react with bisacrylamides, but if the pH of the mixture is increased to above 7.5-8, the reaction takes place. However, their reactivity is low, and PAAs with a fairly high molecular weight are only obtained if the reaction time is very long or the reaction conditions are forced by heating; however, in this case there is a risk of causing hydrolytic degradation reactions at the same time. Glycine is an exception, as it reacts at a rate comparable to that of the majority of amines.